Vital signs of a person, for example the heart rate (HR), the respiration rate (RR) or the blood oxygen saturation, serve as indicators of the current state of a person and can be used as predictors of medical events. For this reason, vital signs are extensively monitored in inpatient and outpatient care settings, at home or in further health, leisure and fitness settings.
One way of measuring vital signs is plethysmography. Plethysmography generally refers to the measurement of volume changes of an organ or a body part and in particular to the detection of volume changes due to a cardio-vascular pulse wave traveling through the body of a subject with every heart beat. Photoplethysmography (PPG) is an optical measurement technique that evaluates a time-variant change of light reflectance or transmission of an area or volume of interest. PPG is based on the principle that blood absorbs more light than surrounding tissue, so variations in blood volume with every heart beat affect transmission or reflectance correspondingly. Besides information about the heart rate, a PPG waveform (also referred to as PPG signal) can comprise information attributable to further physiological phenomena such as the respiration. By evaluating the transmissivity and/or reflectivity at different wavelengths (typically red and infrared), the blood oxygen saturation can be determined. Conventional pulse oximeters are often attached to the skin of the subject. Therefore, they are referred to as ‘contact’ PPG devices.
Recently, non-contact, remote PPG (RPPG) devices for unobtrusive measurements have been introduced. Remote PPG utilizes light sources or, in general radiation sources, disposed remotely from the subject of interest. Similarly, also a detector, e.g. a camera or a photo detector, can be disposed remotely from the subject of interest. Therefore, remote PPG systems and devices are considered unobtrusive and well suited for medical as well as non-medical everyday applications.
Verkruysse et al., “Remote plethysmographic imaging using ambient light”, Optics Express, 16(26), 22 Dec. 2008, pp. 21434-21445 demonstrate that photoplethysmographic signals can be measured remotely using ambient light and a conventional consumer level video camera. One of the main advantages of camera-based vital signs monitoring over on-body sensors is the high ease-of-use: there is no need to attach a sensor, just aiming the camera at the skin/chest of the subject is sufficient. Another advantage of camera-based vital signs monitoring over on-body sensors is the potential for achieving motion robustness: cameras have a significant spatial resolution while contact sensors mostly consist of a single element detector.
Since camera-based vital signs monitoring is performed by carefully analyzing very subtle variations of the skin color, it is highly dependent on the ability of the camera sensor to capture these. One of the key challenges for this technology is to be able to provide robust measurements in low light environments or under varying environment lighting conditions ranging from full sun light to bedroom light levels at night. Hence, the camera sensitivity is a key issue in this process. Usually video signals are captured in the analog domain and then digitized. The signals of interest for RPPG, i.e. the variations in light intensity or brightness, are typically weak (in the order of 0.25 LSB), especially if commercially available cameras are used or if the surrounding lighting conditions are rather weak. Thus, there is a high probability to completely lose the signal, i.e. the relevant information, during the analog-to-digital (AD) conversion step due to the noise in the captured image signal.
In US 2010/0066849 A1 an image processing method and device are described. The presented method and device allow capturing the contents of a scene, determining a binning pattern for pixels representing the scene based on measured brightness values of the pixels and capturing contents of the image using the binning pattern. Such a method may allow reducing camera noise at the cost of a lower resolution and may be used when determining binning settings.
WO 2011/055288 A1 discloses a method of providing a combination of video data and metadata. The method includes obtaining a sequence of images captured by a video camera. At least one signal is extracted from the sequence of images, wherein each extracted signal characterizes local temporal variations in at least one of light intensity and color. At least one video compression technique is applied on image data of images from the sequence to obtain compressed video data The extracted signals are extracted from images in a state prior to the application of the at least one compression technique to image data from those images. The compressed video data is provided with metadata for characterizing at least one process in a subject represented in at least part of the images, which process causes local temporal variations in at least one of color and intensity of light captured from the subject. The metadata is at least based on at least one of the extracted signals
WO 2011/042839 A1 discloses a method of facilitating obtaining a first signal for analysis to characterize at least one periodic component thereof. The method includes obtaining at least two second signals representative of intensities of captured electromagnetic radiation, each corresponding to a respective different radiation frequency range. The first signal is at least derivable from an output signal obtainable by applying a transformation to the second signals such that any value of the output signal is based on values from each respective second signal at corresponding points in time. The method further includes obtaining at least one value of at least one variable determining influences of at least components of respective second signals on the output signal when the signals corresponding to the second signals are captured and the transformation is applied, by at least one of: (i) analyzing at least one of the second signals, an output signal obtained by applying the transformation to the second signals and a first signal derived from the output signal and using the analysis to select at least one value of at least one parameter corresponding to a respective one of the variables; and (ii) calculating values of at least one time-varying factor corresponding to a respective one of the variables, each factor value based on at least one second signal value, and applying each factor in an operation in at least one of a number of parallel sequences of operations comprising at least one such operation and taking a signal corresponding to a respective one of the second signals as input.